Method of heat treating linear long-length steel articles, apparatus for effecting said method and articles produced thereby

ABSTRACT

During heat treatment a steel stock being treated is continually transferred in succession from one technological operation to another with a speed of 10-300 m/min, and temper-heating is performed to a temperature of 300*-700*C at a rate of 50*3000*C/s. In an apparatus for effecting said method arranged intermediate of a last current lead of an austenitizing-temperature heating zone, as viewed in the direction of the stock travel, and a first current lead of a temper-heating zone is a ferromagnetic core made as one or several packs composed of steel plates. Peculiar to an article resulting from the above heat-treating procedures are high physical, mechanical and rheological characteristics.

United States Patent 1 Filatov et al.

[ Dec. 30, 1975 METHOD OF HEAT TREATING LINEAR LONG-LENGTH STEEL ARTICLES, APPARATUS FOR EFFECTING SAID METHOD AND ARTICLES PRODUCED THEREBY [76] Inventors: Nikolai Grigorievich Filatov, ulitsa M. Gorkogo, 41, kv. 78, Orel; Jury Yakovlevich Meshkov, ulitsa Vernadskogo, 75A, kv. 21, Kiev; Nikolai Fedorovich Chernenko, Bulvar Lepse, 48/24,kv 57, Kiev; Dmitry losifovich Nikonenko, ulitsa Dobrokhotova, 7, kv. 156, Kiev; Felix lsaakovich Mashlenko, M. Bronnaya ulitsa, 22/15, kv. 21, Moscow; Albert Mikhailovich Belyaev, ulitsa Bolotnikovskaya, 39, kv. 22, Moscow; Artur Khadyevich Valeev, proezd Rusanova, l 1, korpus 1, kv. 68, Moscow; Valentina Alexandrovna Savina, Denisovsky perevlok, 14, kv. 14, Moscow; Vladimir Zakharovich Marchenko, prospekt Kanatchikov, 6, kv. 55; Valery Ionovich Fedorov, prospekt Kanatchikov, 3, kv. 8, both of Volgograd; Vitaly Nikiforovich Grid'nev, ulitsa K. Libknekhta 36, kv. 15, Kiev; Konstantin Vasilievich Mikhailov, Zelenodolskaya ulitsa, 16, kv. 57, Moscow, all of USSR.

22 Filed: July 26,1973

211 Appl. No.: 382,850

[52] US. Cl 148/150; 148/154; 266/3 R;

266/4 E; 266/5 E; 219/117; 219/118 [51] Int. Cl. C2lD 9/62;C21D l/40 [58] Field of Search 148/143, 150, 154, 153,

148/156; 266/3 R, 4 E, 5 E; 2l9/10.41, 10.57,10.61,117,1l8

[56] References Cited UNITED STATES PATENTS 2,059,054 10/1936 Stargardter 148/156 X 2,224,998 12/1940 Wood et al. 148/154 X 2,932,502 4/1960 Rudd et al. 148/150 X 3,469,829 9/1969 Fujito et a1. 266/3 3,761,323 9/1973 Hunt 148/12 F 3,795,550 3/1974 Ettenreich et a1. 148/150 Primary ExaminerC. Lovell Attorney, Agent, or Firm-Holman & Stern [57 ABSTRACT Peculiar to an article resulting from the above heat-treating procedures are high physical, mechanical and rheological characteristics.

3 Claims, 5 Drawing Figures US. Patent Dec. 30, 1975 Sheet 3 of4 3,929,524

US. Patent Dec. 30, 1975 Sheet40f4 3,929,524

METHOD OF HEAT TREATING LINEAR LONG-LENGTH STEEL ARTICLES, APPARATUS FOR EFFECTING SAID METHOD AND ARTICLES PRODUCED THEREBY BACKGROUND or THE INvENTIoN The present invention relates to a method of heattreating linear, long-length steel articles, such as wire, strip, tubes and special sections, with a view to enhancing their physical and mechanical characteristics.

Known in the art is a method of heat-treating wire, wherein initial steel stock (formed or hot-rolled) is secured between electric contacts and heated to an austenitizing temperature (900-l200C) at a rate of 850-950C/s. Next the stock is cooled to room temperature for obtaining a martensitic structure throughout the entire section, (martensite hardening) and on being then transferred and secured between another pair of electric contacts undergoes a repeated electrical heating to a temperature of 50 0-550C at a rate of 900l0O0C/s to temper the hardened structure.

The steel resulting from the above electrical heating has a structure of thin lamellar sorbite and is suitable for further drawing.

The above-described steel structure enables the production of wire featuring high physical and mechanical characteristics onaccount of heavy total reduction on drawing the metal amounting to 99%.

However, the heating of the wire performed in connection with tempering (further abbreviated as temperheating) in a static state at a rate of 900l0O0C/s is associated with heavyinputs of labor and time for transferring the hardened stock to and securing it between the electric contacts, a feature which diminishes materially the productivity of the heat treatment.

The known method envisages mandatory drawing of the heat-treated stock to ensure the requisite physical and mechanical characteristics of the wire.

.This is attributable to the fact thatthe above-specified tempering point (500-550C) does not allow the obtaining of highstrength (more than 140 kg/mm wire made from steels containing up to 0.5% carbon.

In addition, widely known is a method of heat treating continually moving articles, such as spring wire.

The above method includes austenitizing heating of the moving wire in reverberatory furnaces and quenching in an oil bath. As to tempering of the hardened wire, it-is performed in a lead or salt bath.

With the known method the wire movesv at a rate of m/min, its travel rate being limited by the total length of the technological equipment (a heating furnace, quenching and tempering baths). Striving for higher speeds of the wire travel for ensuring higher production rates results in a sharp increase in overall dimensions of the technological equipment and in the cost of production, being thereby economically inexpedient. Moreover, since higher speeds of movement are inexpedient, neither higher rates of austenitizing and temper-heating can be resorted to nor finished products enhanced. I

In addition, low temper-heating rates do not allow low-carbon steels to be used in this case, reducing thereby the economic efficiency of. production.

I Known in the art are apparatus for resistance heating of wire continually moving intermediate of appropriate contacts, wherein current leads are-grouped according to heating zones and connected in parallel to one power source. The external current leads of the apparatus are grounded for safety reasons.

The apparatus include a cooling means arranged intermediate of the heating zones. In this case current does not flow along the wire moving through the cooling means, insofar as the current leads are under the same potential. The inherent design of such apparatus does not contemplate individual control of parameters (heating temperature and rate) in the heating zones, the control being feasible within certain rather narrow limits only by varying the length of the heating zones which is uneconomical from design, technological and operational aspects and can eventually result in a reduction in the production rates and lower mechanical characteristics of the wire. Meanwhile, in a number of cases, for example, when the range of products treated in an apparatus comprises different steel grades processed in small lots, the apparatus demands frequent readjustments for which purpose it is more expedient from the point of view of saving time that the parameters of the heating zones be controlled individually.

Obviously, one of the possible solutions in this case consists in the use of individual power sources for each I heating zone. However, the apparatus as well as adjathe quality of 9 cent units must be electrically insulated. V

This requires appreciable material expenditures and causes an extra discomfort in apparatus servicing.

In addition, the cooling means are usually made as open baths, wherein the wire is immersed, this resulting in an increase in floor area occupied by the apparatus.

The articles produced by the known method suffer from a number of disadvantages.

For example, a strip of steel with a carbon content of 0.45% subjected to heat treatment quenching followed by subsequent tempering has a strength of not more than kg/mm retaining its elongation per l00-mm length amounting to 8 =5% which leads to an increase in the mass of a part produced from the strip for use in various mechanisms.

Peculiar to spring wire manufactured by heat treatment quenching and tempering from carbon steels is its low relaxation resistance.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for heat-treating linear, long-length steel articles subjected to basic technological operations (austenitizing heating, hardening and temper-heating) with steel stock moving'at such a speed and the'temperheating characteristics employed in the method being such that the entire heat-treating process be appreciably simplified and expedited with the ensuing enhancement in its productivity.

This object is accomplished by heat-treating linear, long-length steel articles, preferably wire, by heating steel stock to an austenitizing temperature, martensite hardening it and subsequent subsequently temper-heating thereof, with the stock moving continually from one technological operation to another, wherein, according to the invention, the wire to be treated iscarried at a rate of 10-300 m/min, the temper-heating being effected to a temperature of 300700C at arate of 505000C/s.

The above method issuitable for treating articles substantially at least 20 m. long having various outlines, such as tubes, wire, strip, angles and other shapes. Depending on the length of the articles being treated, they are on being tempered either coiled or left linear.

The herein-proposed method is most 'suitablefor' treating articles having an infinite length, preferably wire.

It is expedient that the wire being treated be temperand austenitizing-heated by electric current. Performing the heating by employing the electric resistance method is most economical.

At the station preceding the temper heating zone, the wire can be enclosed in a ferromagnetic core.

Inan apparatus for heat-treating linear, long-length steel articles, preferably wire, comprising power sources, current leads arranged in groups in the austenitizing and temper-heating zones, and a cooling means, a ferromagnetic core is located between the last current lead of the austenitizing zone, as viewed in the direction of the wire travel, and the first current lead of the'temper-heating zone and is made as one or several packs composed of 'steel plates.

It is expedient that the cooling means be accommodated within one orseveral packs.

The essence of the herein-proposed invention is as follows.

We have established that in case the stock being treated moves continually at a speed of -300 m/min. in the course of all the technological operations involved (the austenitizing heating, hardening and subsequent temper-heating), the temper-heating procedure can-be performed at a rate of 503000C/s, which, in turn, results in substantial improvement in the quality of the stock being treated owing to its enhanced physicaland mechanical'properties; wire can be used directly after heat treatment thereof, a considerable increase in production rates of the heat-treating process can be attained; and further operations pertaining to cold deformation (drawing)'aimed at further improvement 'of the physical and mechanical characteristics of the stock can be obviated.

BRIEF DESCRIPTION OF FIGS. 1 AND 2 DETAILED DESCRIPTION OF ONE PREFERRED EMBODIMENT Our experiments show that when hardened steel is temper-heated at a high speed, this results in an appreciable enhancement of strength combined with high plasticity numbers. High-speed temper-heating enables the use of low-carbon steels without an ensuing reduction in strength characteristics.

The situation is well illustrated by lines 1 to 6 shown in FIG. 1, indicative of the level of tensile strength obtained for hardened steels with different carbon contents subjected to tempering with different heating rates up to optimum tempering temperatures.

Thus, line 1 corresponds to a temper-heating rate of 50C/s, line 2 to 100C/s, line 3 to l000C/s, line 4 to l500C/s, line 5 to 2000C/s and line 6 to a rate of 3000C/s.

A steel containing 0.2% carbon and tempered at a rate of 3000C/s (line 6) has the same strength as that with 0.8% carbon tempered at a rate of 50C/s (line 1).

FIG. 2 (line 7) shows that the tempering temperature required for obtaining optimum properties in a steel containing 0.7% carbon rises from 580C at a heating rate of 100C/s to 620C at a heating rate of 3000C/s. This runs concordantly with an increase in strength (line 8) and plasticity (line 9). This is attributable to a rise in the temperature of basic processes involved in tempering hardened steel (regions 10 and 11) caused by a restrained diffusion mobility of carbon atoms in the crystalline lattice of martensite and its decomposition products. A delay of recrystallization phenomena in the steel subjected to post-hardening tempering by high-speed heating results in the preservation of a favorable dislocation substructure stipulating additional strengthening and improved plasticity of the treated steel.

The proposed method contemplates heating the elements being treated in any heating devices ensuring the requisite heating rate.

Such devices may be heating muffle furnaces, fluidized bed furnaces, molten metal or salt baths as well as electrical apparatus of various design intended for direct electric heating of the elements.

The use of electric current for heating steel elements is most expedient, insofar as it makes possible the easiest -attainment of the requisite heating rates, electric resistance heating being most efficient.

For accomplishing austenitizing and temper-heating of the wire by the electric resistance method it is necessary to ensure different heating temperatures and rates and, hence, different voltages in corresponding zones, this being responsible for a considerable potential difference which usually takes place between the austenitizing zone, as viewed in the direction of the wire travel, and the temper-heating zone, so that without appropriate measures it might result in the appearance of current in the wire moving through the cooling means.

In order to prevent this, in the proposed apparatus the portion of the wire being treated between the last current lead of the austenitizing zone and the first current lead of the temper-heating zone is enclosed in a ferromagnetic core. This precludes the passage of electric current through the above-specified portion, since the wire incorporated in the core serves as a winding of a choking coil with an ensuing substantial increase in resistance between the austenitizing and temper-heating zones and a reduction of the current flowing along this portion down to an insignificant value.

Thus, it has been established experimentally that when using a ferromagnetic core at a potential difference between the last current lead of the austenitizing zone, as viewed in the direction of the wire travel, and the first current lead of the temper-heating zone amounting to V, the current magnitude did not exceed 10 A (this corresponding to heating by fraction of a degree) and, hence, a possibility appeared to completely obviate heating within the cooling zone and obtain a martensite structure throughout the section of g the stock being treated.

Without the provision of a ferromagnetic core the current magnitude would amount to approximately 600 A with the ensuing considerable heating of the wire (by some 200-250C).

Thus, the austenitizing and temper-heating zones are electrically isolated allowing independent regulation of electrical parameters during the heating of the moving wire, this in turn enabling rapid alteration of the operating conditions within said zones to suit a change in chemical composition of the steel being treated.

The herein-proposed apparatus for heat-treating long-length steel articles features high economic efficiency, insofar as labor input and material requirements for the manufacture of electric-insulation foundations can be diminished materially, electric energy consumption can be reduced by obviating wire heating in a cooling zone, the heat-treating process can be easily automated with the apparatus built in any heattreating flow line.

The ferromagnetic core in the form of a single or several packs allows choosing the cooling zone of a requisite length and, accordingly,-'the magnitude of the current flowing along the wire in the cooling zone.

Since each pack is composed of ferromagnetic insulated plates, a possibility appears to reduce the losses in the ferromagnetic core and to increase inductive reactance of the wire.

The cooling portion which is a pipe line or a number of pipe lines f lled with coolant flowing along them is encompassed by the ferromagnetic core, i.e. the cooling means is accommodated in such a core to save production floor area.

To prevent wire grounding the ferromagnetic core comprises an insulation tubular sleeve.

For a better grounding protection the packs are mounted on additional insulating gaskets.

With the herein-proposed method and apparatus the strength of finished steel articles may be increased by 30-50% and their initial cost reduced by using steels with lower carbon contents. It is also possible to employ alloy steels which are difficult to work.

Moreover, the present method of heat-treating linear, long-length steel articles is very inexpensive and efficient.

The articles subjected to heat treatment by the proposed method feature high strength combined with high plasticity and rheological characteristics.

This is attributable to the shape of cementite inclusions in a steel subjected to electric tempering, said inclusions being at high heating rates not globular in form as is the case with furnace tempering but in the form of long and very thin (about 0.01 p.) lamella arranged in parallel rows.

For a better understanding of the essence of the present invention given hereinbelow are examples illustrating the embodiment of the proposed method.

Example 1 A steel strip measuring 5X30, 5000 m long wound in a coil is subjected to heat treatment. The steel contains 0.48% carbon. To effect the heat-treating operation one end of the strip is secured in a pulling means which makes the strip move continually through heating induction furnaces.

In a first furnace the strip is heated to a temperature of 1000C at a rate of 300C/s and upon leaving the furnace is subjected to quenching in an oil bath. When admitted to a second furnace the strip is temper-heated to a temperature of 440C at a rate of 500C/s. In the course of heat treatment the strip travels at a speed of 50 m/min.

The heat-treated steel strip is reeled in a coil.

After heat'treatment the strip features the following mechanical properties:

tensile stren th 0' 155-160 kg/mm elongation a ter fracture for a -continued Example 2 A wire, 1.3 mm in diameter, reeled in a bundle, 50 kg in weight, produced from a steel containing 0.58% carbon, is subjected to heat treatment. The bundle is mounted on a decoiler, the free end of the wire being pulled intermediate of the electric contacts of an electrio-heating means to be heated to an austenitizing temperature, whereupon the wire passes through a quenching means, a salt bath, and is fastened to a drum of a pulling means.

The pulling means is actuated and draws the wire at a speed of 300 m/min.

The wire moving through the electric-contact means is heated by the resistance method to a temperature of 950C at a rate of 400C/s; upon leaving the above electric-contact means the hot wire is oil quenched in a quenching means and passes further to a salt bath (with a melt temperature of 700C), wherein it is heated to a temperature of 400C at a rate of 250C/s (the bath 7= 165-175 kg/mm 1!: 45-50% a 4% tensile strength reduction of area elongation of a l00-mm specimen Example 3 A wire 4 mm in diameter produced from a steel containing 0.24% carbon is heat treated by using electriccontact heating. A wire bundle is mounted on a decoiler. The free end of the wire is pulled through an electric-contact means for heating to an austenitizing temperature, then through a quenching means, an electric-contact means for temper-heating, whereupon the end of the wire is secured to the drum of a pulling means.

Upon actuating, the pulling means draws the wire at a rate of m/min. The wire is treated by using the following procedures:

The wire is heated for austenitizing to a temperature of 1000C, the quenching process being effected in a water counterflow; temper-heating is performed to a temperature of 350C at a rate of 2500C/s.

After heat treatment, the wire features the following properties: tensile strength cr -135 kg/mm reduction of area 111 40-45%, elongation of a 100-mm specimen 8 8-l0%.

BRIEF DESCRIPTION OF FIGS. 3-5

The present invention will be better understood from a consideration of a detailed description of an exemplary embodiment thereof, taken in conjunction with the accompanying drawings, wherein:

FIG. 3 is a general view of the apparatus;

FIG. 4 is a longitudinal sectional view of a contact roller; and 7 FIG. 5 shows a scaled-up longitudinal sectional view of a ferromagnetic core assembled of a pack composed of steel plates, with a fragmentary cutaway.

DETAILED DESCRIPTION OF ANOTHER PREFERRED EMBODIMENT Referring now to the accompanying drawings, the apparatus comprises means arranged in succession in the direction of the production (heat-treating) process.

Such means are current leads l2 and 13 (FIG. 3) arranged in groups in an austenitizing-temperature heating zone, and current leads l4 and grouped in a temper-heating zone. Arranged intermediate of the last current lead 13 of the austenitiZing-temperature heating zone and the first current lead 14 of the temperheating zone is a ferromagnetic core made as four packs -16 composed of steel plates 17 (FIG. 5).

The packs 16 are mounted on a frame 18 and are separated therefrom with gaskets 19.

The packs l6 incorporate built-in cooling means 20 and are separated by back-up rolls 21 also isolated from the frame 18.

The current leads 12, l3, l4 and 15 are similar in design.-

Each current lead consists of two pairs of contact rollers 22 and 23 between which the wire 25 being treated is held with the aid of a spring 24 (FIG. 4). To decrease sparking on the wire 25, current flowing along the wire is uniformly distributed between the adjacent pairs'of rollers 22 and 23 with the help of an inductance coil 26, the latter allowing its inductive reactance to be varied in the function of electric resistance of the wire portion defined by the above-specified roller pairs (22 and 23).

The contact roller 22 or 23 comprises a disc 27 (FIG. 4) which mounts a replaceable strap 28, the disc being attached to a current-carrying shaft 29. The currentcarrying shaft 29 is mounted in bearings 30 secured in a casing 31.

To feed current to the shaft 29 and reduce friction, the shaft 29 is immersed into a bath 32 filled with a liquid alloy 33 composed of metals with a low melting point, such as Woods alloy, and protected against oxidation.

Connected to thebath 32 is a current-carrying cable 34 coupled with a transformer 35 and a thyristor voltage controller 36.

The inherent design of the pack 16 is such that its plates 17 are clamped with pins 37 (FIG. 5) and insulated from each other with varnish and paint coatings. The pins 37 are insulated from the ferromagnetic core.

Longitudinally and axially of the pack provision is made for an opening to receive concentric pipes 38, 39 and 40 manufactured from Bakelite, non-magnetic steel and ceramics accordingly.

Built into the inner pipe 40 are a coolant supply sleeve 41 and a drain sleeve 42.

To preclude coolant entrainment with the wire provision is made for air seals 43.

The operation of the above-described apparatus will be clear from the following example.

Example 4 Subjected to the heat treatment is a wire 6.0 mm in diameter, produced from steel witha carbon content of C=0.63% and a silicon content of Si 2.1%, all the operations being performed on the present apparatus.

Prior to heat treatment a wire rod 6.5 mm in diameter in bundles 500 kg in weight is mounted on a dewire-drawing machine) with a store. The wire-drawing machine is actuated and draws the wire rod down to a 6.0mm diameter. The drawing operation is performed until the store is filled to capacity. Then the wire-drawing machine is stopped, the end of the wire produced, 6.0 mm in diameter, is welded to an inlet wire piece which upon being passed between the current leads l2 and 13 (FIG. 3) for austenitizing heating, through the packs 16 of the ferromagnetic core incorporating the cooling means 20, through a means 43 (FIG. 5) for blowing off oil, between the current leads l4 and 15 for temper-heating, through a means for cooling finished wire (not shown in the drawing), is fastened to the drum of the pulling means (not shown either in the drawing since not being comprised in the apparatus proper). The power sources 35 and 36 are adjusted for the required heating conditions. (Voltage across the current leads for austenitizing heating is 76 V and across those for temper-heating 31 V).

The pulling means is actuated and after the inlet wire piece has passed the first current lead for austenitizing heating, the wire is subjected to austenitizing heating to a temperature of 950C at a rate of 1000C/s. The wire motion speed is m/min.

After the last current lead of the austenitizing heating zone the wire passes into the first pack 16 of the ferromagnetic core with the cooling means, wherein the wire is cooled with water at a rate of 3000C/s to a temperature exceeding the martensite point by 50C, i.e. down to 300C. Next the wire proceeds into the second, third and fourth packs of the core wherein it is cooled to room temperature in an oil counterflow (being subjected to quenching). As the wire comes out of each pack, oil is blown from it. After that the wire passes to the current leads l4 and 15 and undergoes temper-heating to a temperature of 550C at a rate of l200C/s. Upon being tempered the wire is cooled in air. For obtaining high-strength reinforcement wire, it is shape-formed and then wound in coils.

The above method was utilized for producing a highstrength reinforcement wire (shaped) adapted for reinforcing prestressed reinforced-concrete elements.

The wire featured the following characteristics:

tensile strength a kg/mm elon ation of IOO-mm specimen 8, 5% tensi e strength to conventional (T elastic strength ratio 0.92

tensile strength to yield point ratio 2 to a temperature of 300 to 700C at a rate of 50 to.

3000C/second, said heat treatment being conducted while continually transferring said stock from one heat treating operation to another at a speed of l0-300 m/minute; said method being further characterized in that said steel stock is electrically isolated between the austenitizing and temper-heating zones by passing said steel stock through at least one ferromagnetic core prlses stee strip. means located between the last current lead of the The method of Glam, 1, wherem the Stock austenitizing zone and the first current lead of the tempr'ses Steel wlre' per-heating zone. 5

2. The method of claim 1, wherein the stock com 

1. A METHOD OF HEAT-TREATING LINEAR, LONG-LENGTH STEEL ARTICLES COMPRISING: ELECTRICAL RESISTANCE HEATING STEEL STOCK TO AN AUSTENITIZING TEMPERATURE, COOLING THE SAID STEEL STOCK TOEFFECT MARTENITIZING HARDENING AND SUBSEQUENTLY ELECTRICAL RESISTANCE TEMPER-HEATING SAID STOCK TO A TEMPERATURE OF 300* TO 7000*C AT A RATE OF 50* TO 3000:C SECOND, SAID HEAT TREATMENT BEING CONDUCTED WHILE CONTINUALLY TRANSFERRING SAID STOCK FROM ONE HEAT TREATING OPERATION TOANOTHER AT ASPEEDOF10-300 M MINUTE; SAID METHODBEING FIRTHER CHARACTERIZED IN THAT SAID STEEL STOCK IS ELECTRICALLY ISOLATED BETWEEN THE AUSTENITIZING AND TEMPER-HEATING ZONES BY PASSING SAIS STEEL STOCK THROUGH AT LEAST ONE FERROMAGNETIC CORE MEANS LOCATED BETWEEN THE LEAST CURRENT LEAD OF THE AUSTENITIZING ZONE AND THE FIRST CURRENT LEAD OF THE TEMPER-HEATING ZONE.
 2. The method of claim 1, wherein the stock comprises steel strip.
 3. The method of claim 1, wherein the stock comprises steel wire. 